1. Field of the Invention
The present invention relates to an information recording medium which is advantageously employed for recording and/or reproducing optical information of signals by means of a laser beam of high energy density. The invention further relates to a method of optically recording information on the information recording medium and a method of optically reproducing the recorded information from the information recording medium.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and are now put into practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file. Among these media, a compact disc (referred to as "CD") is now widely used as an audio disc for reproducing a music or the like.
The compact disc generally comprises a disc-shaped transparent substrate of plastic material which has been beforehand provided with pits having information in the form of EFM (Eight to Fourteen Modulation) digital audio signals, a light-reflecting thin film of Al or other metal having a high reflectance which is arranged on the substrate and a protective film coated over the reflecting thin film. Reading of information from CD is conducted by irradiating CD with a laser beam, and under irradiation the signals of CD format are read out based on the variation of reflectance caused by the presence or absence of the pits.
CD is generally rotated at a fixed linear speed of 1.2 to 1.4 m/sec. according to the predetermined CD standard to record an information thereon, and CD is required to allow a long recording time (at longest 74 min.) using a signal-recording area within the region between inner diameter of 45 mm and outer diameter of 116 mm under the conditions of pit width of 0.8 .mu.m and track pitch of 1.6 .mu.m. The conventional audio compact disc is used only for reproducing information. Such compact disc has been beforehand provided with pits on the substrate (accordingly not provided with a recording layer), so that the compact disc has such problems that information cannot be recorded or edited thereon on the users' side. Accordingly, development of a compact disc of DRAW (Direct Read After Write) type (i.e., disc capable of being written) is required.
The above requirement is also addressed to discs for memorizing various documents, data, static image files, etc., discs of DRAW type for CD-ROM (Read Only Memory) or CD-I (Interactive).
A known information recording medium of DRAW type basically comprises a disc-shaped transparent substrate made of plastic or glass material and a recording layer made of metal or semi-metal such as Bi, Sn, In and Te provided on the substrate. Writing of information on the recording medium (optical disc) can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam and a rise of temperature locally occurs, and as a result, a chemical and physical change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the disc with a laser beam. The recorded information can be reproduced by detecting a reflected light or a transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
In such known information recording media of DRAW type, however, the information cannot be recorded unless the medium is rotated at a high linear speed to have wide spaces between adjoining pits during the recording procedure, and hence it is almost impossible to record information thereon according to the CD system in which high density recording is made at a low linear speed. Even if the medium would record information thereon, the recorded information could not be read out therefrom by means of a commercially available CD player, because the commercially available CD player can read only an optical disc having a high reflectance for a reproducing light such as a laser beam.
As described above, writing of information on the optical disc or reading of information from the optical disc is generally conducted by irradiating a predetermined portion of the disc surface with a laser beam. Recently, as a large amount of information is desired to be recorded on the optical disc, it has been proposed to increase density of the units for record of information in the form of such as pits in recording area of the disc. However, the increase of density of the recording units causes difficulty in precisely irradiating the predetermined portion of the disc with a laser beam, so that optical discs in recent years are generally provided on their surfaces with a tracking guide (also referred to as "pre-groove") having such a sectional form as shown in FIG. 1 of attached drawings for precisely guiding the laser beam to the predetermined irradiation area (generally referred to as "tracking").
The conventional information recording medium of DRAW type is prepared in accordance with the standard defined by ISO (International Organization for Standardization). The depth and width of a groove are defined to give excellent properties to the medium, as described in a publication of "Data on Optical Disc Standardization" (Optical Industry Promotion Association, compiled by Optical Disc Meeting, p. 113, published in Dec. 3, 1987). As to the depth and width of the groove, the above-mentioned publication teaches as follows.
In the case that information is recorded within a groove, the depth of the groove is not less than 700 angstroms and the width of the groove is not less than 0.8 .mu.m. When information is recorded on a land portion (i.e., between grooves), the depth thereof is not less than 800 angstroms and the width thereof is not more than 0.6 .mu.m. In those cases, the optical disc shows excellent properties.
Accordingly, the recording of information is made utilizing a groove having the above-mentioned size, and generally the recording is made on the land portion rather than within the groove because the land portion has a higher reflectance than the groove, that is, a difference of the amount of a reflecting light between an area having a pit and an area not having a pit is larger in the land portion than within the groove.
In a conventional CD, grooves having the above-mentioned size are generally provided. However, it has been confirmed by the present inventor that even in the case of recording information through forming pits on the recording layer by irradiating the above-defined groove on the recording layer with a laser beam, the recorded information can be hardly reproduced using a commercially available CD player which is used for reading information by a three-beam tracking method.
In more detail, when information is recorded on the recording layer having the above-mentioned grooves by irradiating the grooves with a laser beam and forming pits thereon, each of the pits is apt to enlarge (or spread) in the radial direction and the size of the pit becomes too large, whereby occurrence of errors increases in the reproduction of information signals, or tracking precision tends to lower. The reason is assumed that the optical disc of CD format is recorded with information of high density at a lower linear speed as compared with the conventional optical disc, so that heat is easily stored in the disc to bring about the above-described pit enlargement. If the recording power is reduced for avoiding such pit enlargement, there arises other problem that the recording of information is not made completely. The pit enlargement is markedly observed in the case of forming pits on the land portion where any wall does not exist on the edge, but also observed in the case of forming pits within the groove.